1. Field of the Invention
The present invention relates to a lock-up clutch mechanism used in a torque converter of an automatic transmission for a vehicle and more particularly, it relates to an improvement in a frictional sliding surface of the lock-up clutch mechanism.
2. Related Background Art
A torque converter used in an automatic transmission can provide smooth start, acceleration and deceleration, but has worse transmitting efficiency since it transmits a power via fluid. Thus, there has been proposed a technique in which, if a speed of a vehicle exceeds a predetermined value, a lock-up clutch mechanism is operated to connect to a drive wheel directly in order to reduce energy loss and enhance a fuel consumption rate.
Further, in recent years, in order to further enhance the fuel consumption rate, the lock-up clutch mechanism has been operated even at a low speed operation of the vehicle. In this case, in order to reduce engine vibration and transmission shock, so-called slip lock-up control-in which the lock-up control is performed while maintaining a slip amount at a predetermined number of revolutions is carried out.
In general, the lock-up clutch may cause self-excited vibration called as judder, thereby worsening the comfort of the vehicle considerably. The judder is greatly influenced particularly by unbalance of surface pressure distribution in a circumferential direction at a friction surface during the slipping. The unbalance of the surface pressure distribution is greatly influenced by accuracy of the friction surface, and, for example, dispersion is generated on the friction surface in the circumferential direction by undulation of a piston of the lock-up clutch and/or distortion of a fitting bolt for a drive plate provided in a front cover.
In order to suppress of occurrence of such judder, for example, Japanese Patent Application Laid-open No. 2004-011710 proposes a technique in which a difference in density between a radial inner side and a radial outer side of a friction material secured to a lock-up piston by compressing the friction material or reduction of a μ-V property of the friction surface is suppressed by providing a tapered friction material.
However, since the occurrence of the judder greatly relies upon the unbalance of the surface pressure distribution in the circumferential direction at the friction surface during the slipping, the judder is not necessarily avoided by the technique disclosed in the above-mentioned Japanese Patent Application Laid-open No. 2004-011710.